Hydrogen burns to make water only. It can also be combined with oxygen in a fuel cell to produce electricity. Water is, however, a greenhouse gas.

Also, the energy used to produce the hydrogen is from burning hydrocarbons in power plants.

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Alkanes and Alkenes (cont)

Biofuels are made from plants, which absorb carbon dioxide from the atmosphere as they grow, and then release it when the biofuel is burned.

Omitting the energy requirements for the manufacturing process, biofuels are more carbon neutral than coal, oil and gas.

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Alkanes and Alkenes (cont)

Alkenes are unsaturated hydrocarbons, they contain C=C double bonds.

The C=C consists of a sigma and a pi bond.

In sigma bonds the electron cloud is concentrated between 2 nuclei (all single bonds).

In pi bonds, the electron cloud is above and below the plane of the molecule.

The pi bond does not allow rotation.

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Alkanes and Alkenes (cont)

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Naming Geometric Isomers

Geometric isomers occur due to the lack of rotation around a C=C double bond.

Different groups can therefore be arranged on different sides of the molecule.

E-Z system is used to name geometric isomers.

Each group is ranked by atomic number (higher atomic number, higher rank).

An 'E' isomer has the two highest ranked groups on opposite sides.

A 'Z' isomer has the two highest ranked groups on the same side.

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Naming Geometric Isomers (cont)

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Reactions of Alkanes

When burned in a limited supply of oxygen, alkanes undergo incomplete combustion and form carbon monoxide and water (oxygen is still O2).

Carbon monoxide is odourless, invisible and toxic- it can be fatal.

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Reactions of Alkanes (cont)

In a substitution reaction, one atom/group is replaced with another atom/group.

Chlorine will replace hydrogen in methane to form chloromethane.

This is free-radical substitution.

A free-radical is a species with an unpaired electron, such as Cl.

This is formed from the dissociation of chlorine at about 300'C or in UV light.

The free-radical subsitution by chlorine is a 3 step mechanism.

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Reactions of Alkanes (cont)

Initiation- UV light provides energy to break Cl-Cl bond, generating 2 free-radicals. The electron pair in the covalent bond is split so each Cl takes an electron with it. This is called homolytic fission.

Propagation- a Clreacts with a hydrogen in methane to form HCl and leaves a CH3 free-radical.The CH3 reacts with chlorine to produce chloromethane, leaving a Clfree radical.

Termination- the combination of any of the free-radicals with eachother or with themselves produces stable molecules, eliminating the free radicals.

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Reactions of Alkenes

The C=C double bond in alkenes makes them very reactive, because they can react across the double bond- an addition reaction.

This forms a single, saturated product.

In catalytic hydrogenation, hydrogen is added to an alkene to produce an alkane at 200'C in the presence of a high surface area nickel catalyst.

When an alkene is reacted with acidified potassium manganate, the alkene is oxidised as as addition takes place, forming a diol.

potassium manganate changes purple to colourless.

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Reactions of Alkenes (cont)

When bromine approaches the C=C double bond, its electron cloud shifts due to the repulsion from the dense C=C; producing an instantaneous dipole.

The +ve part of the bromine acts as an electrophile and attracts an electron pair from the double bond, forming a C-Br bond on a carbocation, and leaving a bromide ion with an electron pair.